Interplant movement of Heliothis virescens (Lepidoptera: Noctuidae) larvae in pure and mixed plantings of cotton with and without expression of the Cry1Ac δ-endotoxin protein of Bacillus thuringiensis Berliner

Abstract

Laboratory and field studies were conducted during 1993 and 1994 to quantify interplant movement of Heliothis virescens (F.) larvae in pure and mixed plantings of cotton, Gossypium hirsutum L., with ('Event 531') and without ('Coker 312') the expression of Cry1Ac δ-endotoxin protein of Bacillus thuringiensis Berliner. Field studies were conducted with neonate, 4-, and 7-d-old larvae placed on 3-plant experimental units and observed at 24, 48, 72, and 96 h after inoculation of larvae. Combining larval movement across observations of neonates, 4-, and 7-d-old larvae, an estimated 52% of the larvae on pure plantings of Coker 312 had moved at least 1 plant by the cumulative time required to reach the age of 10 d. More larvae placed on Event 531 cotton moved to an adjacent plant (13% of the neonates had moved at least I plant within 24 h) than those placed on Coker 312 (0% of the neonates had moved at least I plant within 24 h). When larvae were placed on Event 531 plants, an estimated 82% of the larvae had moved to an adjacent plant by cumulative age of 10 d. Collectively, these data indicate that movement of larvae from plant to plant increases with larval age and occurs more rapidly for larvae placed on Event 531 cotton than on Coker 312. Previous studies have suggested that resistance to B. thuringiensis could develop more rapidly in insects exposed to seed mixtures of plants with and without endotoxin if larvae move between plants and if an external refuge exists. These data provide evidence of larval movement between plants in seed mixtures.